1. Field of the Invention
This invention relates generally to implantable medical devices and, more particularly, to methods, apparatus, and systems for affecting neuromodulation using an external evoking signal and/or an internal evoking signal to control a therapeutic stimulation signal applied by a medical device.
2. Description of the Related Art
The human nervous system (HNS) includes the brain and the spinal cord, collectively known as the central nervous system (CNS). The central nervous system comprises nerve fibers that transmit nerve signals to, from, and within the brain and spinal cord. The network of nerves in the remaining portions of the human body forms the peripheral nervous system (PNS). Some peripheral nerves, known as cranial nerves, connect directly to the brain to control various brain functions, such as vision, eye movement, hearing, facial movement, and feeling. Another system of peripheral nerves, known as the autonomic nervous system (ANS), controls blood vessel diameter, intestinal movements, and actions of many internal organs. Autonomic functions include blood pressure, body temperature, heartbeat and essentially all the unconscious activities that occur without voluntary control.
Like the rest, of the human nervous system, nerve signals travel up and down the peripheral nerves, which link the brain to the rest of the human body. Many, but not all, nerve fibers, in the brain and the peripheral nerves are sheathed in a covering called myelin. The myelin sheath insulates electrical pulses traveling along the nerves. A nerve bundle may comprise up to 100,000 or more individual nerve fibers of different types, including larger diameter A and B fibers which comprise a myelin sheath and C fibers which have a much smaller diameter and are unmyelinated. Different types of nerve fibers, among other things, comprise different sizes, conduction velocities, stimulation thresholds, and myelination status (i.e., myelinated or unmyelinated).
More generally, the endogenous electrical activity (i.e., activity attributable to the natural functioning of the patient's own body) of a neural structure of a patient may be modulated in a variety of ways. In particular, the electrical activity may be modulated by exogenously applied (i.e., from a source other than the patient's own body) electrical, chemical, or mechanical signals applied to the neural structure.
The modulation of neural activity (hereinafter referred to generally as “neurostimulation” or “neuromodulation”) may involve the application of an exogenous signal for the induction of afferent action potentials, efferent action potentials, or both, in the neural structure, and may also involve blocking or interrupting the transmission of endogenous electrical activity traveling along the nerve. Electrical neurostimulation or modulation of a neural structure refers to the application of an exogenous electrical signal (as opposed to a chemical or mechanical signal), to the neural structure. Electrical neurostimulation may be provided by implanting an electrical device underneath the skin of a patient and delivering an electrical signal to a nerve such as a cranial nerve. The electrical neurostimulation may involve performing a detection, with the electrical signal being delivered in response to a detected body parameter. This type of stimulation is generally referred to as “active,” “feedback,” or “triggered” stimulation. Alternatively, the system may operate without a detection system once the patient has been diagnosed with epilepsy (or another medical condition), and may periodically apply a series of electrical pulses to the nerve (e.g., a cranial nerve such as a vagus nerve) intermittently throughout the day, or over another predetermined time interval. This type of stimulation is generally referred to as “passive,” “non-feedback,” or “prophylactic,” stimulation. The stimulation may be applied by an implantable medical device that is implanted within the patient's body.
The state-of-the-art also provides various methods for applying external signals to portions of a patient's body. An “external” signal, which may comprise an evoking signal and/or a therapeutic stimulation signal, refers to a signal delivered to the patient's body from a source outside the patient's body. The externally originating signal may, however, achieve its functional (e.g., stimulating and/or evoking) effect upon the target portion of the patient's body either internally or externally. For example, transcranial magnetic stimulation, an external signal, may be provided for internally stimulating various portions of the brain and/or cranial nerves. Further, external electrical stimulation may be provided by electrodes coupled to the patient's body externally. For example, electrodes coupled to various parts of a patient's face may stimulate various portions of the trigeminal nerve.
For many patients, the placement of the IMD, the associated electrodes and leads, in a particular location of the body may determine the efficacy of the neurostimulation delivered by the IMD. In some patients, neurostimulation of the left vagus nerve may be less effective than the neurostimulation provided to the right vagus nerve, or vice versa. This knowledge may be helpful in determining whether to implant the IMD on the right side or the left side of the patient, or on both left and right sides. The state-of-the-art generally lacks an effective manner of determining which side of a patient's body would be more conducive to stimulation from an IMD. More generally, effective screening methods are needed to assist in identifying, prior to implantation, those patients who are potential responders to neurostimulation therapy.
Additionally, different patients may exhibit dissimilar reactions to stimulation provided by the IMD. Generally, physicians estimate the value of various parameters (e.g., such as the amplitude, pulse widths, time periods between stimulation, etc.,) when determining the type of therapeutic signal to be used for stimulation. These estimates may be based upon prior experience with other patients and may not be optimum for other patients. Therefore, the physicians have to wait to analyze the results of the neurostimulation before assessing whether a current set of stimulation parameters are appropriate for a particular patient. However, analyzing the results of a therapeutic stimulation regimen may be a long-term process when studying the effectiveness of the treatment. Valuable treatment opportunities, and time, may be lost due to a time-lag in determining whether particular stimulation parameters being employed in an IMD is indeed effective. Therefore, the recovery or the attenuation of a particular disorder may be delayed using the state-of-the-art processes for determining the effectiveness of a particular set of stimulation parameters.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.